A Fibre Bragg Grating (FBG) is a permanent, photo-induced periodic modulation of the refractive index in the core of an optical fibre, which reflects light within a narrow bandwidth centred at the Bragg wavelength. It is known that FBGs, through measurements of the Bragg wavelengths, can be used to detect any perturbations, such as temperature or strain (at the locations of the FBGs), which changes the physical period of the refractive index modulation and/or the effective refractive index seen by the propagating light along the FBG, and hence the Bragg wavelength. An important feature of FBG sensors is that the reflected Bragg wavelength from an FBG sensing element, with a proper calibration, is an absolute measure of the magnitude of the perturbation(s) at the FBG. However, in sensing applications measuring for example temperatures and strains, the Bragg wavelength has to be measured with a resolution, repeatability and accuracy about 1 pm, which is typically only 0.3-1% of the FBG reflection bandwidth. With a typical Bragg wavelength of 1.55 .mu.m a Bragg wavelength shift of 1 pm corresponds to a change in strain and temperature of approximately 1 m strain and 0.1.degree. C., respectively.
Another very important feature of FBG sensors is that many FBG sensors can be multiplexed along one or several fibres by writing the FBGs at different wavelengths which do not overlap under sensor operation, to perform quasi-distributed sensing with the FBG sensing elements placed at arbitrary positions separated by a few millimeters up to tens of kilometers.
It is known that one or several reflected FBG sensor wavelengths can be measured using a broadband source, for example an edge-light-emitting diode (ELED) or a superfluorescent fibre source (SFS), in combination with a tuneable optical filter, for example a piezoelectric transducer (PZT) tuneable fibre Fabry-Perot filter [Kersey, A. D., Berkoff, T. A., and Morey, W. W., "Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter", Optics Letters, Vol. 18, pp. 1370-1372, 1993], or alternatively a tuneable laser source, for example an external cavity semiconductor laser with an external tuneable FBG reflector [U.S. Pat. No. 5,401,956 (Mar. 28, 1995)], provided the source spectrum covers all possible FBG sensor wavelengths. These techniques enable simultaneous wavelength demultiplexing and demodulation (wavelength determination) of the various FBG sensors. The transmission wavelength of the tuneable filter (or the laser source) will normally be scanned over the complete wavelength range of the sensors, where the control voltages to the tuning element, or the scan times, corresponding to maxima in the detected power are measures of the sensor Bragg wavelengths. The relationship between the control voltage, or scan time, and the tuning wavelength, i.e. the filter response, will for practical tuning elements not be linear, and will suffer from drift and hysteresis in the filter response. This is particularly true for a PZT-driven tuning element. This implies that some sort of reference scheme is required to measure Bragg wavelengths with high accuracy and repeatability.
Another problem when using broadband sources and tuneable filters for wavelength demodulation of FBG sensors is the effect of gradients and ripples in the filtered source spectrum (i.e. in the real source spectrum and/or the filter transmission spectrum). This can cause small wavelength shifts in the measured peak wavelengths relative to the real Bragg wavelengths of the FBG sensors, which will vary with changing positions of the Bragg reflection spectra relative to the filtered source spectrum, and lead to uncertainties in measured Bragg wavelength.
Wavelength division multiplexing (WDM), using several closely spaced laser transmitter wavelengths, is becoming increasingly important in fibre optic telecommunications to exploit the huge bandwidth provided by optical fibre transmission and enhance the information transmission capacity. In such applications it is essential to set and control the wavelengths of the transmitters with a high degree of accuracy and repeatability. This is also very often the case for the laser wavelength of tuneable lasers, which have many applications in component characterisation, spectroscopy and sensing applications.
Finally, in measuring the reflected wavelengths from FBG sensors with closely spaced Bragg wavelengths, or analysing the various wavelength channels in a densely spaced WDM system in terms of transmitter wavelengths and signal-to-noise ratio, using an instrument based on a tuneable filter, there might be crosstalk between the various sensors/channels caused by the sidebands in the filter transmission spectrum.